Provide Point+Size, Point-Point, Point.toSize(), and Rect.toPoint().

sky/engine/core/painting/Point.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 part of dart.sky;
 
 /// Holds 2 floating-point coordinates.
 class Point {
   final double x;
   final double y;
 
   const Point(this.x, this.y);
 
-  bool operator ==(other) {
-    if (!(other is Point)) return false;
-    return x == other.x && y == other.y;
-  }
+  bool operator ==(other) => other is Point && x == other.x && y == other.y;
+  Size operator -(Point other) => new Size(x - other.x, y - other.y);
+  Point operator +(Size size) => new Point(x + size.width, y + size.height);
+
+  // does the equivalent of "return this - Point(0,0)"
+  Size toSize() => new Size(x, y);
+
   int get hashCode {
     int result = 373;
     result = 37 * result + x.hashCode;
     result = 37 * result + y.hashCode;
     return result;
   }
   String toString() => "Point($x, $y)";
 }